Molecular Testing Accuracy for Clostridium difficile Scrutinized

The selection of diagnostic tests to confirm CDI is controversial because of the variety of laboratory methods that are available and used across various facilities, and lack of standard recommendations for testing. Although amplification of the toxin B gene by polymerase chain reaction (PCR) is a sensitive method for detecting toxigenic C. difficile, false negative results still occur and could impact the diagnosis and treatment of this infection.

Scientists at the Northern Ontario School of Medicine (Sudbury, ON, Canada) and their colleagues tested stool samples from patients with diarrhea, and suspected of having CDI, were tested in the diagnostic laboratory for C. difficile using a PCR test and culture techniques. Multiplex PCR was performed on presumptive C. difficile isolates to detect the Toxin A gene (tcdA), Toxin B gene (tcdB), and the triose phosphate isomerase gene (tpi). The reactions were carried out in an Eppendorf Mastercycler nexus thermal cycler (Fisher Scientific, Ottawa, ON, Canada).

A total of 2,308 samples were tested for toxigenic C. difficile by GeneXpert C. difficile epi test (Cepheid, Sunnyvale, CA, USA) and culture methods over a 15-month period. The observed agreement of the results of the two tests was 95.1%. A total of 295 samples tested positive by GeneXpert, while 2,013 samples were negative by GeneXpert. C. difficile isolated from the discrepant samples resulted in diverse ribotyping patterns suggesting they were derived from different strains. The samples belonged to patients who were distributed evenly between age groups and wards in the hospital. In the majority of cases, the false negative C. difficile test results did not seem to impact the clinical outcome in these patients.

The authors concluded that although molecular methods are considered among the most sensitive methods available for clinical diagnosis of CDI, it seems inevitable that some cases, which are positive for C. difficile, might be missed when using this method, or any other method for that matter. Both clinical and analytical sensitivity of C. difficile tests should be considered when deciding which diagnostic assay to use, and clinical correlates should be examined carefully before excluding CDI as a cause of disease. The study was published on August 19, 2016, in the journal BMC Infectious Diseases.

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Northern Ontario School of Medicine
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